Structured thermoplastic in composite interleaves

ABSTRACT

One or more layers of structured thermoplastic polymer are located within the interleaf zones of a preform or prepreg. The structured thermoplastic polymer is a film or membrane formed by: a) dissolving polyethersulfone or polyetherimide in a solvent to form a polymer dope; b) dispersing a curing agent for an epoxy resin in the polymer dope to form a curative-enriched polymer dope; c) casting the curative-enriched polymer dope onto a surface to form a dope layer; and d) removing the solvent from the dope layer to form a layer of structured thermoplastic polymer.

This application is a continuation-in-part of U.S. Ser. No. 14/884,484,filed on Oct. 15, 2015, which is a continuation-in-part of U.S. Ser. No.13/699,035, filed on Nov. 20, 2012, which is a 371 of PCT/IB2011/000728filed on Apr. 4, 2011, and which also is a continuation-in-part of U.S.Ser. No. 14/133,965, filed Dec. 19, 2013, now U.S. Pat. No. 9,492,971,issued Nov. 15, 2016, which is a divisional of U.S. Ser. No. 12/225,280,filed Sep. 17, 2008, which is 371 of PCT/GB2007/001079, filed Mar. 23,2007.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to thermoplastic toughening materials foruse in composite assemblies. The present invention also relates topreforms which are infused with thermosetting resin and to prepregcomprising fibres and thermosetting resin, both of which are cured toform a composite material. More particularly, one embodiment of thepresent invention involves the use of a polyethersulfone film ormembrane as a structured thermoplastic material that is present in thelaminate interleaves of the composite material.

2. Description of Related Art

Composite materials made from a polymeric matrix and a reinforcingfibrous material are used in many commercial applications includingaerospace, sports goods, transportation, civil engineering and energygeneration. Some of the most commonly used composites are made fromthermosetting resins with glass or carbon-fibre reinforcement. Epoxyresin are the most extensively used class of matrix polymers for highperformance applications, followed by phenolics, cyanate esters,bismaleimides, benzoxazines and several less familiar chemistries. Thesecombinations of fibre and curable polymeric matrix resins are used in anincreasing variety of processes, according to the requirements of thespecific application. Particularly convenient ways of utilizingcomposites are prepreg, semi-preg, resin transfer moulding (RTM), vacuumassisted resin transfer moulding (VaRTM), liquid resin infusion (LRI),resin film infision (RFI), pultrusion, pressure assisted moulding andseveral variants thereof.

The choice of a specific type of composite process is influenced by manyfactors including cost, convenience, complexity of the part to be made,health and safety considerations and mechanical performancerequirements, In general, the highest performance can be achieved by thejudicious assembly of layers of prepreg. This is a labour intensiveoperation and it is preferable to use RTM or some other resin injectionor infusion method. In RTM, an infusible structure (or preform) is madefrom reinforcing fibres and other additive including binders, and thepreform is injected or infused with liquid resin and the resin is curedat elevated temperature to form a useable component.

Prepregs, comprising a layer of fibre impregnated with resin such asepoxy resin, are widely used in the generation of such compositematerials. Typically a number of plies of such prepregs are “laid-up” asdesired and the resulting laminate is cured, typically by exposure toelevated temperatures, to produce a cured composite laminate.

However, although such cured materials have a number of clear benefits,it has long been known that they can suffer from poor impact resistanceand be prone to delamination. This is particularly the case when epoxyresin systems are used, which are known to tend to produce cured systemswith low toughness.

A widely employed method of improving the toughness of such arrangementsis for the laminate of a plurality of prepreg fibre layers to beinterleafed with resin layers. Commonly such resin interleaf layers alsocomprise a distribution of thermoplastic toughener particles. Thisarrangement has been shown to increase the toughness of the laminatewithout having a detrimental effect on other aspects of the laminate.

Laminates that have interleaf layers toughened with thermoplasticparticles are typically cured under autoclave conditions, where the hightemperatures, and more importantly the high pressures, are generallyrequired to provide cured laminates that meet the particularly exactingmechanical specifications required for structural applications.

A widely used alternative to autoclave cure is the so-called vacuum bagor out-of-autoclave cure. This utilizes a vacuum and relies onatmospheric pressure to press down onto the laminate during cure.Although much more economical than autoclave curing, the maximumpressure applicable in out-of-autoclave curing is atmospheric pressure.Laminates that have interleaf layers toughened with thermoplasticparticles have typically not been cured outside of an autoclave becausecuring at atmospheric pressure or below tends to produce cured laminatesthat have unacceptable mechanical properties for many structuralapplications including aerospace structural applications.

It would therefore be desirable to develop a prepreg which could be usedto produce laminates that can be cured, either inside or outside of anautoclave, to provide composite parts that are sufficiently tough to besuitable for structural application including aerospace structuralapplications.

Manufacture of composite assemblies falls into two broad categories,namely direct and indirect. Direct manufacture allows a cured compositeassembly to be produced without intermediate product being formed. Incontrast, indirect manufacture produces an intermediate prepreg whichmay be partially cured. The cured composite assembly is then producedoff-line.

Whilst prepregs remain important to the industry, there is a generalmove towards direct manufacturing processes such as RTM and RFI. Directprocesses are generally preferred as the problems associated with thestorage of prepregs are eliminated. Furthermore, direct processes reducewaste and reduce both manufacturing times and costs as the need for anextra impregnation step is eliminated. Direct processes also allow morecomplex composite parts to be manufactured.

SUMMARY OF THE INVENTION

The present invention seeks to provide a film or membrane which findsutility in both direct and indirect manufacturing processes, whereinsaid film or membrane serves to toughen the composite assembly of whichit forms a part. The invention has particular, but not exclusiveapplication in direct manufacturing processes for composites, such asRTM and RFI.

According to one aspect of the present invention, there is provided anon-fibrous membrane or film, as embodied in Example 8, comprising atleast one soluble thermoplastic polymeric material, said material beingat least partially soluble in a monomer and insoluble in a polymerderived from the monomer.

In accordance with another aspect of the present invention, uncuredlaminates are provided where an uncured thermosetting resin and aplurality of fibrous layers are combined such that the fibrous layersare separated by an interleaf zone located between adjacent fibrouslayers. As a feature of the invention, one or more layers of structuredthermoplastic polymer, such as a film or membrane, veil of thermoplasticfibers or other similar open-structured sheet, are located within one ormore of the interleaf zones. The layers of structured thermoplasticpolymer are from 0.5 to 50 microns thick and have a weight per unit areaof from 1 to 20 grams per square meter.

The use of one or more layers of structured thermoplastic polymer in theinterleaf zone not only toughens the cured laminate, but also provides anumber of advantages over the conventional use of thermoplasticparticles as the interleaf toughening agent. For example, it wasdiscovered that the use of one or more layers of structuredthermoplastic polymer as the interleaf toughening agent gives one theoption to cure the laminate at relatively low pressures without reducingtoughness, as is the case with particulate interleaf tougheners. Inaddition, two layers of different structured thermoplastic polymers maybe located within a single interleaf zone to provide a structuredorientation of different thermoplastic polymers that is not possiblewith a mixture of randomly oriented dissimilar particles. Further, useof structured thermoplastic polymers in the interleaf zone eliminatesthe problems associated with particulate tougheners that may includesome particles that migrate during cure to locations, both inside andoutside of the interleaf zone, where their effectiveness may be reduced.

The invention covers the prepreg that is used to make uncured laminateswhere one or more layers of structured thermoplastic are located withinthe laminate interleaf zones. Such prepreg include those where thefibrous layer is sandwiched and held between layers of structuredthermoplastic and those where one or more layers of structuredthermoplastic polymer are located on one side of the fibrous layer.

The invention covers methods for making prepreg and methods for usingthe prepreg to make laminates. In addition, methods for making curedparts from the prepreg and laminates, as well as the final cured parts,are also covered by the invention.

In one aspect, the invention relates to a prepreg comprising astructural layer of fibres and an open-structured sheet, the prepregbeing impregnated with curable resin comprising thermosetting resin.Another aspect of the invention involves preforms that comprisestructural layers of fibres and layers of structured thermoplasticpolymer.

Such a prepreg, taken alone or when laid together with a plurality ofsimilar prepregs and cured forms a composite laminate having excellenttoughness properties, even when the convenient out-of-autoclave curecycle is employed, and can also achieve the high fibre volumes requiredin structural applications.

The improved prepregs of the present invention may be used in a widevariety of applications where a lightweight but structurally toughlaminate is needed. However, they are particularly useful in aerospaceapplications, where the technical requirements are particularlyexacting.

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional view of two exemplary types ofprepreg in accordance with the present invention.

FIG. 2 is a simplified cross-sectional view of an exemplary laminatemade from one of the exemplary prepregs shown in FIG. 1.

FIG. 3 is a simplified cross-sectional view of an exemplary laminatemade from one of the exemplary prepregs shown in FIG. 1.

FIG. 4 is a top view of a preferred exemplary thermoplastic veil for usein the laminate interleaf in accordance with the present invention(scale=cm).

FIG. 5 is a magnified (80×) top view of the preferred exemplarythermoplastic veil shown in FIG. 4.

FIG. 6 is a diagrammatic view showing an exemplary process for making ahybrid layer of structured thermoplastic polymer in which a veil ofpolyamide fibers are coated with polyethersulfone.

FIG. 7 shows the polyamide veil used in the process shown in FIG. 6prior to its being coating with polyethersulfone.

FIG. 8 shows the polyamide veil of FIG. 7 after it has been coatedaccording to the process shown in FIG. 6 to form a hybrid layer ofstructured thermoplastic polymer.

DETAILED DESCRIPTION OF THE INVENTION

The prepreg and laminates of the present invention may be used in placeof existing prepreg and laminates where thermoplastic particles, whichare insoluble in the resin matrix, are located in the interleaf betweenfibre plies to increase the damage tolerance of the cured compositepart. Such prepreg and laminates are used to form interleaf-toughenedcomposite parts for structural applications in the aerospace industryand in any other application where high structural strength and damagetolerance is required. The present invention involves substituting oneor more layers of structured thermoplastic polymer or open-structuredsheet into the laminate interleaf zone in place of the thermoplasticparticles to provide a number of advantages that are not possible usingthermoplastic particles. Accordingly, the present invention may beapplied to any of the known manufacturing and curing processes wherethermoplastic particles are located in the laminate interleaf as atoughening agent.

The structured layers of thermoplastic polymer are preferably used as areplacement for substantially all (95 weight percent or more) of thethermoplastic particles that would otherwise be placed in the interleafzones of the laminate. However, mixed systems are possible wherethermoplastic particles and structured thermoplastic layers are combinedin the interleaf zone. Preferably, the majority (50 weight percent ormore) of the insoluble thermoplastic in the interleaf zone will be inthe form of one or more structured layers of thermoplastic polymer.

A simplified cross-sectional view of a preferred exemplary prepreg isshown at 10 in FIG. 1. The prepreg 10 includes a single layer ofunidirectional fibres 12 which is sandwiched between two layers ofstructured thermoplastic polymer, such as polyamide veils 14 and 16. Theprepreg 10 includes a thermosetting resin, which may be included as oneor more layers (not shown) or it can be infused or impregnatedthroughout all or part of the prepreg 10.

A simplified cross-sectional view of a second preferred exemplaryprepreg is shown at 20 in FIG. 1. The prepreg 20 includes a single layerof unidirectional fibres 22 and a single layer of structuredthermoplastic polymer, such as polyamide veil 24. The prepreg 20includes a thermosetting resin, which may be included as one or morelayers (not shown) or it can be infused or impregnated throughout all orpart of the prepreg 20.

An exemplary laminate is shown at 30 in FIG. 2 where three layers ofprepreg 10 have been stacked to form a 3-layer laminate where there aretwo structured thermoplastic polymer layers 14 and 16 located in theinterleaf zones 32 located between each fibrous layer 12. Anotherexemplary laminate is shown at 40 in FIG. 2 where three layers ofprepreg 20 have been stacked to form a 3-layer laminate where there isone structured thermoplastic polymer layer 24 located in the interleafzones 42 located between each fibrous layer 22. Only three layers areshown in the figures for demonstrative purposes. The laminate mayinclude many more layers depending upon the design parameters for theparticular composite part being made. In addition, two or more layers ofstructured thermoplastic polymer may be used in place of the singlestructured layers shown at 14, 16 and 24. Typical laminates comprisefrom 4 to 200 layers of structural fibres with most or all of the layersseparated by a curable thermosetting resin interleaf layer. Suitableinterleaf arrangements are disclosed in EP0274899.

The material that is used to form the structured thermoplastic polymerlayers 14, 16 and 24 must meet a number of criteria in order for thelayers to function properly as a replacement for the insolublethermoplastic particles that previously have been used in the interleafzones. The structured polymer layers can be made from the samethermoplastic polymers that have been used to make thermoplastictoughening particles. In general, the thermoplastic must be insoluble inthe matrix resin (typically an epoxy) at room temperature and at theelevated temperatures used to cure the resin. Depending upon the meltingpoint of the thermoplastic polymer, it may melt or soften to varyingdegrees during curing at elevated temperatures and re-solidify as thecured laminate is cooled. Suitable thermoplastics for use in making thestructured layer should not dissolve in the resin, as is the case withepoxy-soluble thermoplastics, such as polyethersulfone (PES) andpolyetherimide (PEI).

Suitable examples of thermoplastic polymers that may be used to form thestructured polymer layers are polyamides (PA), copolyamides (CoPA),ether or ester block polyamides (PEBAX, PEBA), polyphthalamide (PPA),polyesters such as polyethylene terephthalate (PET) and polybutyleneterephthalate (PBT), copolyesters (CoPE), thermoplastic polyurethanes(TPU), polyacetals, polyolefins, polyphenylenes sulfones,polyetheretherketones (PEEK), polyetherketoneketone (PEKK),poly(phenylene sulfate) (PPS), thermoplastic polyimides (PI),polyamideimide, liquid crystal polymers (LCP), block copolymers such asstyrene-butadiene-methylmethacrylate (SBM) copolymers,methylmethacrylate-acrylate of butyl-methylmethacrylate (MAM)copolymers.

Polyamides and copolyamides are the preferred thermoplastic materialsfor the structured thermoplastic layers. Particles made from polyamidesand copolyamides have been a used as interleaf toughening particles inthe past (See U.S. Pat. No. 7,754,322 and published U.S. PatentApplication No. 2010/0178487A1). Polyamides come in a variety of types,such as caprolactame (PA6), laurolactame (PA12), copolymers of PA6 andPA12, as well as PA 10 and 11. Any of the polyamides that are suitablefor making particles that are used to toughen laminate interleaf zonesare also suitable for use in making layers of structured thermoplasticpolymer in accordance with the present invention.

Structured polyamide layers will have different melting temperaturesdepending upon the particular polyamides used to make the layer, thedegree of copolymerization of the polyamide and the degree ofcrystallinity. For example, polyamide layers that contain mostlypolyamide 6 will typically have a melting point of above 190° C.Polyamide layers made from PA6 alone will typically have melting pointson the order of 213° C., whereas structured layers made from a copolymerof 80 weight percent PA6 and 20 weight percent PA 12 will have meltingpoints on the order of 194° C. When the polyamide copolymer contains 20weight percent PA6 and 80 weight percent PA12, the melting point dropsto around 160° C. Structured polyamide layers made from PA12 alonetypically have even lower melting points.

As an advantage of the present invention, the structured thermoplasticlayers 14 and 16 or 24 may be made from different types of polyamides.This allows one to mix and match layers of polyamide, or other suitablethermoplastic polymer, within the interleaf zones of the laminate. Sincethe thermoplastic layers are structured, the placement of eachparticular type of polyamide can be carefully controlled. This type ofdirected polyamide placement is not possible with polyamide particleswhere a mixture of different particle types results in a homogenousblend within the interleaf zone.

As an example, thermoplastic polymer layer 14 would be a veil ofPA6/PA12 copolymer fibers and thermoplastic polymer layer 16 would be aveil of PA12 fibers. The resulting laminate 30 would include interleafzones 32 in which discreet layers of PA6/PA12 and PA112 are located.There are many other combinations possible depending upon the desirednumber of different thermoplastic polymers, the number of layers placedin each interleaf zone, the orientation and order of prepreg stacking toform the laminate and the orientation of the thermoplastic layers in theprepreg.

The structured thermoplastic polymer layers must be in a physical formthat allows them to be substituted in place of particles in theinterleaf zone. In particular, the layers must be sufficiently thin tofit within the interleaf zone and the density of the layer must be suchthat the appropriate amount or concentration of thermoplastic materialis present in the interleaf zone to impart the desired amount of damagetolerance. Interleaf zones in cured high strength structural laminatestypically have a thickness that ranges from 10 to 100 microns. Preferredinterleaf zones range in mean thickness from 15 microns to 50 microns.

The structured thermoplastic layer should have a thickness that allowsit to fit within the above interleaf zones along with a thermosettingresin. When more than one structured thermoplastic layer is to belocated within an interleaf zone, as is the case with the laminate shownin FIG. 2, the combined thickness of the layers should be less than thedesired interleaf zone thickness of the cured laminate. The thickness ofthe structured layer(s) should be no more than 95 percent of the desiredinterleaf zone thickness. Preferably, the thickness of the structuredlayer(s) will not be more than 75 percent of the desired interleaf zonethickness.

The thickness of the structured thermoplastic layers should be from 0.5microns to 50 microns. The actual thickness for each layer will bedetermined by the intended thickness of the interleaf zone in the curedlaminate, the number of layers desired in each interleaf zone, thedensity of the structured thermoplastic layer and the amount(concentration) of thermoplastic polymer that is desired within eachinterleaf zone. The preferred thickness range for the structuredthermoplastic layers is from 2 microns to 35 microns and most preferablyfrom 3 microns to 20 microns.

The density of the structured thermoplastic layer must be such that itprovides the desired amount (concentration) of thermoplastic toughenerto the interleaf zone. The needed density for the layer is directlydependent upon the thickness of the layer being used. The thinner thelayer, the denser the layer must be in order to provide the sameconcentration of thermoplastic toughener in the interleaf zone. Thedensity of the structured thermoplastic layer should be such that itprovides a structured layer that has an areal weight of 1 to 20 gramsper square meter (gsm) for layers that range from 0.5 to 50 microns inthickness. For preferred thermoplastic layers that are 2 to 35 micronsthick, it is preferred that the density of the thermoplastic layer besuch that the areal weight of the layer is from 2 gsm to 10 gsm. Forlayers that are from 3 to 20 microns thick, the density of the layershould be such that the areal weight of the layer is from 2 gsm to 8gsm.

Structured thermoplastic polymer layers having the required combinationof thickness and areal weight are available commercially in the form ofspunlaced and random fibrous veils. Woven fabric or scrim is generallynot suitable due to the relatively lightweight and thin structurerequired for the structured thermoplastic layer. Thin solid or porousfilms are also not preferred. An exemplary lightweight (4 gsm) fibrousveil is shown in FIG. 4 and FIG. 5 (80× magnification). The veil is madefrom randomly oriented PA12 fibers and is available as 128D04 Nylon veilfrom Protechnic (Cernay, France). Another suitable nylon veil is 128D06Nylon veil, which is a 6 gsm PA12 fibrous veil that is also availablefrom Protechnic.

Fibrous veil of the type shown in FIGS. 4 and 5 are generally classifiedby the percentage of openness. For example, 128D04 Nylon fibrous veilhas an openness of 70 percent. The heavier 128D06 Nylon fibrous veil hasan openness of 50 percent. The structured thermoplastic polymer layershould have an openness of between 30 percent to 98 percent, preferablyfrom 50 percent to 95 percent and more preferably from 60 percent to 90percent. This percentage of openness is required in order to providefree passage of resin through the structured thermoplastic to insurecomplete mixing of the resin with the thermoplastic fibers. This isparticularly important since the thermoplastic fibers do not dissolve inthe resin during cure.

The structured thermoplastic layer may also be viewed as anopen-structured sheet that is an integral sheet held together byinterconnecting and/or overlapping fibres. Such fibres may be woven,knitted, also be random e.g. spunlaced or laid scrim, althoughnon-woven, e.g. random is preferred. Such a sheet is often referred toin the art as a veil.

The open-structured sheet may be characterized by the degree of opennessof the sheet, i.e. the percentage of average surface area of the sheetwhich is made up of open holes in the sheet as determined on an areabasis by image analysis of micrographs. The open-structured sheets ofthe present invention typically have a degree of openness of from 30% to98%, preferably from 50% to 95%, more preferably from 60% to 90%. Thishelps to maintain a lightweight sheet and also allows the free passageof resin.

The structural fibres 12 or 22 may be in the form of random, knitted,non-woven, multiaxial or any other suitable pattern. For structuralapplications, it is generally preferred that the fibres beunidirectional in orientation as shown in FIGS. 1-3. The laminates inFIGS. 2 and 3 show the various layers of unidirectional fibres orientedin the same direction. This is only one of many possible orientationsfor stacks of unidirectional fibre layers. For example, unidirectionalfibres in neighboring layers may be arranged orthogonal to each other ina so-called 0/90 arrangement, which signifies the angles betweenneighboring fibre layers. Other arrangements, such as 0/+45/−45/90 areof course possible, among many other arrangements. The laminates shownin FIGS. 2 and 3 are in a 0/0/0 orientation. When unidirectional fibrelayers are used, the orientation of the fibre will typically varythroughout the laminate stack.

The structural fibres 12 or 22 may comprise cracked (i.e.stretch-broken), selectively discontinuous or continuous fibres. Thestructural fibres may be made from a wide variety of materials, such ascarbon, graphite, glass, metalized polymers, aramid and mixturesthereof. Carbon fibres are preferred. Typically the fibres in thestructural layer will generally have a circular or almost circularcross-section with a diameter in the range of from 3 to 20 μm,preferably from 5 to 12 μm.

Exemplary layers of unidirectional fibres are made from HexTow® carbonfibres, which are available from Hexcel Corporation (Dublin, Calif.).Suitable HexTow® carbon fibres for use in making unidirectional fibrelayers include: IM7 carbon fibres, which are available in fibres thatcontain 6,000 or 12,000 filaments and weigh 0.223 g/m and 0.446 g/m,respectively; IM8-IM10 carbon fibres, which are available in fibres thatcontain 12,000 filaments and weigh from 0.446 g/m to 0.324 g/m; and AS7carbon fibres, which are available in fibres that contain 12,000filaments and weigh 0.800 g/m.

An added advantage of the invention is that the structured thermoplasticmay be used to hold dry unidirectional fiber together so that it can beprocessed using relatively simple prepreg processing equipment, such asthat used for making fabric prepreg by film transfer where the fibrehandling aspect of the machine is much simpler than typicalunidirectional fiber prepreg machines.

As a feature of the present invention, it was discovered that thelightweight fibrous veils and other similar structured thermoplasticlayers that can be used as a substitute for thermoplastic interleaftoughening particles, also provide an effective means for holdingunidirectional fibre layers together during handling prior to prepregformation or other resin infusion. The fibrous veils function both as atemporary holding system for the unidirectional fibres and asthermoplastic toughening agent for the cured laminate.

It is preferred that the fibrous veil and similar structuredthermoplastic layer be selected so that it provides the desired level ofstability to the unidirectional fibre layers during initial handling ofthe dry unidirectional fibers and that it also provides the desiredlevel of laminate interleaf toughening. It was found that the sandwichconfiguration shown at 10 in FIG. 1 is a preferred orientation forachieving these two goals. The location of a single lightweight fibrousveil on either side of dry unidirectional fibres was found to providesufficient holding power to keep the dry unidirectional fibres togetherduring handling. When the laminate was formed (FIG. 2), the singlelayers of fibrous veil located on opposite sides of the unidirectionalfibre layer were combined in the interleaf zone 32 to form a doublelayer of fibrous thermoplastic material. This double layer of fibrousthermoplastic material was found to provide a sufficient amount ofthermoplastic in the interleaf zone to provide desired levels oflaminate toughening.

The two layers of fibrous veil are made from randomly orientedthermoplastic fibres. Accordingly, they are preferably attached to thedry unidirectional fibers by partially melting or softening the veilsand simultaneously pressing the veils against the unidirectional fibres.The partially melted/softened fibres bond to the unidirectional fibrelayer and provide stabilization of the layer when the veils are cooledback below their melting temperatures. The stabilized dry unidirectionalfibre layer or tape is then ready for handling or storage prior toaddition of resin to form the prepreg 10. This type of sandwichconfiguration is useful in situations where the addition of resin toform the prepreg is to take place at a later time or different location.

Fibrous veils of thermoplastic material, which have the properties setforth above, are uniquely suited and preferred for use in accordancewith the present invention. When appropriately placed, they are able tofunction as both a substitute for conventional stabilization systems fordry unidirectional fibres and as a substitute for thermoplasticparticles used to toughen laminate interleaf zones. Appropriateplacement is required in order to achieve the above-described holdingfunction. For example, it was found that a single layer of fibrous veilof the type suitable for use as an interleaf toughener, when bonded toonly one side of dry unidirectional fibres, is not sufficient to holdthe dry fibres together during subsequent handling of the dry fibres.Accordingly, it is necessary to provide additional support to the dryunidirectional fibres or employ a process where the resin and fibrousveil are simultaneously applied to the unidirectional fibers to form aprepreg of the type shown at 20.

Typically the fibres 12 and 22 of the prepregs 10 and 20, respectively,will be substantially impregnated with a thermosetting resin (notshown). For example, prepregs with a resin content of from 30 to 45 wt %of the total prepreg weight are preferred. The prepregs of the presentinvention are predominantly composed of resin and structural fibres.Typically the prepreg comprises from 25 to 50 wt % of curable resin.Additionally the prepregs typically comprise from 45 to 75 wt % ofstructural fibres.

The resin in the prepreg is also preferably present in an amount thatforms a resin-rich region on the surface of the prepreg that is anessentially fibre-free layer adjacent to the structural fibre layer.When a plurality of such prepregs is laid together, the fibre-free resinlayers form the interleaf layers between the structural fibre layers.

As discussed above, the prepregs according to the invention are intendedto be laid up with other prepregs, to form a curable stack of prepregs.Thus, one aspect, the invention relates to a curable stack of prepregs,the stack comprising a plurality of layers of structural fibres and aplurality of curable thermosetting resin interleaf layers substantiallyfree of structural fibres, wherein at least one interleaf layercomprises at least one structured thermoplastic layer. Typically, mostof the interleaf layers will comprise a structured thermoplastic layeror open-structured sheet. In a preferred embodiment at least half of theinterleaf layers comprise an open-structured sheet. It may even bedesirable for at least 75% of the interleaf layers to comprise such asheet, or even substantially all of the interleaf layers as shown inFIGS. 2 and 3.

Typically, the fibres in the prepreg stack will be substantiallyimpregnated with the resin. For example, prepreg stacks with a resincontent of from 30 to 45% of the total weight of the prepreg stack orlaminate are preferred.

As discussed above, in the eventual cured composite laminate, theopen-structured polymeric sheet is located at or in the interleaf layer.However, during the heating stage prior to cure, the thermosetting resinhas a reduced viscosity which tends to encourage the movement of theopen-structural sheet into the interleaf layer. Thus, it is onlynecessary in the prepreg or prepreg stack for the open-structured sheetto be in contact with the resin layer, and not necessarily embeddedtherein.

It has been found that the improvements in toughness can be achievedeven though the structured thermoplastic polymer layer oropen-structured sheet is very lightweight. This is particularlyimportant for aircraft structural applications. Thus, open-structuredsheets having weights per unit area in accordance with the presentinvention, as set forth above, are particularly well-suited foraerospace applications.

In the preferred embodiment shown at 10, the prepreg comprises twoopen-structured sheets located on either side of the structural layer offibres. This can aid in handling the prepreg, particularly prior toresin impregnation and can provide further increases in toughness.Preferably the two sheets are substantially identical. However, they mayalso be made from different thermoplastic polymers to provide specifictargeting of different toughening agents within the interleaf zone.

The prepreg and prepreg stack of the present invention typicallycomprise a very low quantity of entrapped gas so that the degree ofresin impregnation in the interstices of the structural fibres is high.Thus, they preferably have a water pick-up value of less than 9%, morepreferably less than 6%, most preferably less than 3%. The water pick-uptest is well known in the art and involves immersing an edge of a smallpiece of unidirectional prepreg into water.

The prepreg is intended to be laid-up with other composite materials(e.g. other prepregs according to the invention or otherwise) to producea curable laminate or prepreg stack according to the present invention.

The prepreg is typically produced as a roll of prepreg and in view ofthe tacky nature of such materials, a backing sheet is generallyprovided to enable the roll to be unfurled at the point of use. Thus,preferably the prepreg according to the invention comprises a backingsheet on an external face.

The curable resin may be selected from epoxy, isocyanate, benzoxazine,bismaleimide and acid anhydride, for example. Preferably the curableresin is an epoxy resin.

Suitable epoxy resins may comprise monofunctional, difunctional,trifunctional and/or tetrafunctional epoxy resins.

Suitable difunctional epoxy resins, by way of example, include thosebased on; diglycidyl ether of bisphenol F, diglycidyl ether of bisphenolA (optionally brominated), phenol and cresol epoxy novolacs, glycidylethers of phenol-aldelyde adducts, glycidyl ethers of aliphatic diols,diglycidyl ether, diethylene glycol diglycidyl ether, aromatic epoxyresins, aliphatic polyglycidyl ethers, epoxidized olefins, brominatedresins, aromatic glycidyl amines, heterocyclic glycidyl imidines andamides, glycidyl ethers, fluorinated epoxy resins, glycidyl esters orany combination thereof.

Difunctional epoxy resins may be preferably selected from diglycidylether of bisphenol F, diglycidyl ether of bisphenol A, diglycidyldihydroxy naphthalene, or any combination thereof.

Suitable trifunctional epoxy resins, by way of example, may includethose based upon phenol and cresol epoxy novolacs, glycidyl ethers ofphenol-aldehyde adducts, aromatic epoxy resins, aliphatic triglycidylethers, dialiphatic triglycidyl ethers, aliphatic polyglycidyl ethers,epoxidised olefins, brominated resins, triglycidyl aminophenyls,aromatic glycidyl amines, heterocyclic glycidyl imidines and amides,glycidyl ethers, fluorinated epoxy resins, or any combination thereof.Suitable trifunctional epoxy resins are available from Huntsman AdvancedMaterials (Monthey, Switzerland) under the tradenames MY0500 and MY0510(triglycidyl para-aminophenol) and MY0600 and MY0610 (triglycidylmeta-aminophenol). Triglycidyl meta-aminophenol is also available fromSumitomo Chemical Co. (Osaka, Japan) under the tradename ELM-120.

Suitable tetrafunctional epoxy resins include N,N,N′,N′-tetraglycidyl-m-xylenediamine (available commercially fromMitsubishi Gas Chemical Company under the name Tetrad-X, and as ErisysGA-240 from CVC Chemicals), andN,N,N′,N′-tetraglycidylmethylenedianiline (e.g. MY 0720 and MY0721 fromHuntsman Advanced Materials). Other suitable multifunctional epoxyresins include DEN 438 (from Dow Chemicals, Midland, Mich.), DEN 439(from Dow Chemicals), Araldite ECN 1273 (from Huntsman AdvancedMaterials), and Araldite ECN 1299 (from Huntsman Advanced Materials).

The curable resin may also comprise one or more curing agent. Suitablecuring agents include anhydrides, particularly poly carboxylicanhydrides; amines, particularly aromatic amines e.g.1,3-diaminobenzene, 4,4′-diaminodiphenylmethane, and particularly thesulphones and methylene bisanilines, e.g. 4,4′-diaminodiphenyl sulphone(4,4′ DDS), and 3,3′-diaminodiphenyl sulphone (3,3′ DDS),4,4′-methylenebis (2-methyl-6-isopropylaniline (M-MIPA),4,4′-methylenebis (3-chloro-2,6-diethylene aniline (M-CDEA),4,4′-methylenebis (2,6 diethyleneaniline) (M-DEA) and thephenol-formaldehyde resins. Preferred curing agents are the methylenebisanilines and the amino sulphones, particularly 4,4′ DDS and 3,3′ DDS.

The prepregs according to the present invention can be manufactured in avariety of ways. For example, the structural fibres may be brought intocontact with the structured thermoplastic polymer layer oropen-structured sheet and then, whilst in contact, are together passedto an impregnation stage where at least one layer of resin is broughtinto contact with an external face of the fibre and open-structuredsheet (structured thermoplastic polymer layer) combination, and pressureapplied to induce resin impregnation. Alternatively the open-structuredsheet (structured thermoplastic polymer layer) can be applied to theresin layer, and thereafter the structural fibre layer is brought intocontact with the resin and open-structured sheet (structuredthermoplastic polymer layer) combination, before pressure-induced resinimpregnation occurs. As a further alternative the structural layer maybe resin impregnated without the open-structured sheet (structuredthermoplastic polymer layer), which is subsequently laid-down onto anexternal surface of the resin-impregnated structural layer.

However, due to their light and delicate nature, the structuredthermoplastic polymer layers or open-structured sheets used in thepresent invention can be difficult to handle, particularly if they areto be laid onto a tacky resin surface. Thus, it has been found to bepreferable if the structured thermoplastic polymer layer is laid downonto a resin-free surface.

Thus, in another aspect, the invention relates to a process for themanufacture of a prepreg, the process comprising feeding a structurallayer of fibres in contact with an adjacent structured thermoplasticpolymer layer or open-structured sheet, and bringing into contact withan external face of the structural layer and/or the structuredthermoplastic polymer layer (open-structured sheet) a layer of curableresin comprising thermosetting resin, and compressing the resin, fibresand sheet together, sufficient to induce at least partial resinimpregnation into the interstices between the structural fibres.

As mentioned previously, it is advantageous for the structural layer offibres to be sandwiched between two adjacent open-structured sheets(structured thermoplastic polymer layer) prior to resin impregnation, asthis helps to maintain the integrity of the fibres, particularly whenthe structural fibres are unidirectional. In a preferred process, thefibres of the open-structured sheet are adhered to the fibres bypartially melting them.

In order to increase the rate of impregnation, the process is preferablycarried out at an elevated temperature so that the viscosity of theresin is reduced. However it must not be so hot that premature curing ofthe resin begins to occur. Thus, the process is preferably carried outat temperatures of from 40° C. to 100° C.

The resin is typically spread onto the external surface of a roller andcoated onto a paper or other backing material to produce a layer ofcurable resin. The resin can then be brought into contact, andoptionally also impregnated, by passing the structural layer,open-structured sheet (structured thermoplastic polymer layer) andresin-coated paper through rollers. The resin may be present on one ortwo sheets of backing material, which are brought into contact with thestructural layer and open-structured sheet (structured thermoplasticpolymer) by passing them through heated consolidation rollers toimpregnate.

If a backing sheet is to be applied then this can be carried out eitherbefore or after impregnation of the resin. However, it is typicallyapplied before or during impregnation as it can provide a non-sticksurface upon which to apply the pressure required for resinimpregnation. Typically the backing sheet is the one on which the resinwas mounted, although it can be removed and replaced with a differentsheet as desired.

Once prepared the prepreg is typically rolled-up, in which form it canbe stored for a period of time. It can then be unrolled and optionallylaid up with other prepregs to form a prepreg stack as defined herein.

Once prepared, the prepreg or prepreg stack is cured by exposure toelevated temperature, and optionally elevated pressure, to produce acured laminate. As discussed above, the prepregs of the presentinvention can provide excellent toughness without requiring the highpressures encountered in an autoclave process.

Thus, in further aspect, the invention relates to a process of curing aprepreg or prepreg stack as described herein, the process involvingexposing the prepreg to a temperature sufficient to induce curing andcarried out at a pressure of less than 3.0 bar absolute.

The curing process may be carried out at a pressure of less than 2.0 barabsolute. In a particularly preferred embodiment the pressure is lessthan atmospheric pressure. The curing process may be carried out at oneor more temperatures in the range of from 80 to 200° C., for a timesufficient to cause curing to the desired degree.

Curing at a pressure close to atmospheric pressure can be achieved bythe so-called vacuum bag technique. This involves placing the prepreg orprepreg stack in an air-tight bag and pulling a vacuum on the inside ofthe bag. This has the effect that the prepreg stack experiences aconsolidation pressure of up to atmospheric pressure, depending on thedegree of vacuum applied.

Once cured, the prepreg or prepreg stack becomes a cured compositelaminate, suitable for use in a structural application, for example anaerospace structure.

Such composite laminates can comprise structural fibres at a level offrom 55% to 70% by volume, preferably from 58% to 65% by volume.

The present invention has particular application as an alternative toepoxy-based prepreg where the insoluble interleaf toughening agent isprovided as resin-insoluble thermoplastic particles. For example, seeU.S. Pat. No. 7,754,322 B2 and WO 2008/040963. These types of epoxyresins that are used to form toughened interleaf zones typically includea soluble thermoplastic toughening agent, such as polyethersulfone orpolyetherimide. These soluble toughening agents are included in amountsthat range from 5 to 25 weight percent of the overall resin composition.The soluble toughening agents are typically added to the epoxy resinmixture prior to addition of the curing agent and heated to an elevatedtemperature to dissolve the thermoplastic curing agent and then cooled.Insoluble thermoplastic particles, the curing agent and any otheradditives are added to the resulting mixture and then used incombination with fiber layers to form prepreg. The insolublethermoplastic particles are typically added in amounts of between 1 and15 weight percent of the overall resin composition.

During prepreg and laminate formation, as well as curing of thelaminate, the insoluble particles, which generally have mean particlesizes between 5 and 60 microns, become concentrated in the interleafzones and other areas outside of the structural fibre layers. This isbecause the substantial majority of insoluble particles are too large toenter into the interstitial openings of the fibre layer. Due toprocessing and other manufacturing considerations, the particle powdersthat are used as the insoluble thermoplastic tougheners may have smallamounts of particles that are substantially smaller or larger than thetarget size range. The smaller particles present a problem in that theycan migrate into the fibre layers during laminate formation and curingwhere their effectiveness as an interleaf toughener is diminished. Thelarger particles present a problem with respect to possible disruptionof the interleaf zone during curing of the laminate due to theirrelatively large size.

The present invention involves providing structured thermoplasticpolymer layers that are uniformly thick and contain insoluble fibersthat cannot possibly enter into the structural fiber layers. The layerthicknesses and densities are chosen so that the amount of insolublethermoplastic toughener located within the interleaf zone falls withinthe same range as is provided by using the above-described resins thatcontain insoluble thermoplastic particles. The present inventionprovides the dual advantage of making sure that all of the insolublethermoplastic toughener that is present in the prepreg remains in theinterleaf zones of the laminates while at the same time insuring thatthe interleaf zone is not disrupted due to variations in thermoplasticmaterial sizes and shapes.

The unique properties of lightweight veils of thermoplastic fibres, andother similar structured thermoplastic polymer layers, make it possibleto cure the laminates using such veils in an out-of autoclave process.This relatively low pressure and low cost curing process can be usedbecause the damage tolerance (e.g. Compression After Impact—CAI) of thecured laminate is not substantially less than the damage toleranceachieved using the higher pressure and higher expense of an autoclave.In contrast, out-of-autoclave curing of laminates that have interleafzones toughened with insoluble thermoplastic particles produces curedlaminates that have damage tolerances that are significantly reduced.

For structural uses in aerospace and other high tolerance applications,it is preferred that a laminates in accordance with the presentinvention comprising 32 plies of 145 gsm fibre areal weight prepreg in aquasi-isotropic stack arrangement have a CAI value at 30 kJ (accordingto AITM 1.0010 or EN6038) of greater than 250 MPa, preferably greaterthan 300 MPa.

One embodiment of the invention involves a hybrid structuredthermoplastic layer or hybrid open-structured sheet in which a solublethermoplastic material is coated on or otherwise secured to a substrate.The substrate used in this hybrid structured thermoplastic layercomprises a non-woven material. Hence the substrate ideally comprisesany of the following either alone or in combination: aramid, glass,ceramic, hemp, polyamide or polyolefin. Polyamide or carbon fibers areparticularly useful for forming the substrate. One suitable substrate isa fibrous Nylon veil commercially available from Protechnic of Franceand is of 5 gsm weight.

This so-called hybrid membrane ideally has a weight per unit area in therange from 1 to 25 gsm and/or a thickness in the range from 5 μm to 25μm.

The hybrid structured thermoplastic layer can be applied to a prepreg asa separate layer, either next to the prepreg's reinforcement or on theresin surface or otherwise used in a direct process such as within anRTM preform. The hybrid can be applied prior, during or after theimpregnation process for standard film impregnation of woven orunidirectional fibres.

A preferred, but not exclusive application is in direct compositemanufacturing processes, particularly those comprising unidirectionalreinforcements.

In this hybrid embodiment of the invention, the thermoplastic is ideallyheat treated in order to reduce its speed of dissolution in the matrixresin.

The thermoplastic material may be applied to the substrate in anydesired pattern.

An exemplary process for making a hybrid structured thermoplastic layeris shown in FIG. 6. FIG. 6 shows processing equipment 30 whereby a 5 gsmveil 31 of non-woven polyamide material is fed from a roll 32 over acoating roller 33 which coats the veil 31 with a polymer dope from apolymer dope reservoir 34. The polymer dope comprises a mixture of DMSO(dimethyl sulfoxide). PES (polyether sulfone) and Orgasol™ polyamide.Support is provided for the veil via upper and lower polythene webs 35and 36. The coated veil is fed through a coagulation bath 37 andsubsequently through two wash baths 38 and 39 charged with water. Thewet coated substrate then passes to a storage roller 40. The roll 40 issubsequently passed through vertical ovens (not shown) to provide afinished dried product attached to the polythene webs which are easilyremovable.

The final product, not including the polyethylene webs, weighedapproximately 6 to 7 gsm and contained about 2-3 gsm of PES.

FIG. 7 shows the polyamide veil material that was used as the substratein the process illustrated in FIG. 6. Here veil fibrous elements can beseen. Comparing with FIG. 8, showing a micrograph of the coated veil,solidified PES can be observed as the transparent material between theveil fibrous elements. Likewise, undissolved Orgasol, which providesadditional toughening to the cured laminate, can be observed as smalldark dots.

FIG. 8 shows the dried PES thermoplastic material coated on thepolyamide veil. This exhibits discrete pore arrangements.

Another embodiment of a structured thermoplastic layer in accordancewith the present invention involves the formation of film ofthermoplastic polymer that includes an epoxy and a curative for theepoxy as set forth in Example 8.

The invention will now be illustrated by reference to the followingexamples.

Example 1 (Prepreg A)

A sheet of 145 gsm IM7-12K UD fibre surrounded on both sides by anopen-structured sheet (4 gsm (128D04 from Protecnic, France)) was made.A prepreg was made from this open-structured sheet and UD fibre byapplying to either side an epoxy-based M56 resin film (a mixture ofMY721 epoxy resin (available from Huntsman) with dissolvedpolyethersulphone and methylene bisaniline curative) of 36 gsm andpassing through consolidation rollers to form a prepreg. The resultantprepreg had a resin content of 32%.

Comparative Example 2 (Prepreg B)

A comparative prepreg was manufactured as in Example 1 using UD fibrebut without the veil to form a prepreg of the same areal weight and witha resin content of 35%.

Example 3 (Prepreg C)

A prepreg was manufactured by applying 36 gsm M56 resin films to eitherside of 134 gsm AS7-12K UD fibre and passing through consolidationrollers. Subsequently, 128D04 veil was then applied to one side of theprepreg before passing through a further set of consolidation rollers.The resultant prepreg according to the invention had a resin content of35%.

Comparative Example 4 (Prepreg D)

A comparative prepreg was made according to Example 3 but without theopen-structured sheet and had a resin content of 35%.

Comparative Example 5 (Prepre E)

A modified M56 resin was produced by adding during mixing, 10% Orgasol1002 DNATL particles (20 micron PA6) available from Arkema. A prepregwas made from this modified M56 resin by applying 39 gsm film eitherside of 145 gsm UD IM7-12K fibres and passing through consolidationrollers to form a prepreg. The resultant prepreg had a resin content of35%.

Comparative Example 6 (Prereg F)

A modified M56 resin was produced by adding during mixing, 10% Micropan777 particles (7 micron PA6) available from Chemopharma, Czech Republic.A 35% resin content prepreg with 145 gsm IM7-12K fibres was made in thesame way as Example 5.

Manufacture of Composite Laminates

Prepregs A-F, were used to manufacture 32 ply quasi-isotropic laminatesof size 400×400 mm. The plies were debulked every four plies. Thelaminates were cured in a vacuum bag inside an air-circulating ovenaccording to the following cure cycle.

-   -   ramp to 110° C. at 1° C./min    -   dwell 110° C. for 60 minutes    -   ramp 1° C./min to 180° C.    -   dwell 180° C. for 120 minutes.

The vacuum level was reduces to half vacuum (−0.5 bar) after the end ofthe 110° C. dwell. Prior to that vacuum level was greater than −0.9 bar

The laminates produced were designated laminates A-F, according to theircorresponding prepregs.

Laminate Thickness

Laminate thickness and cured ply thickness are shown in the Table Ibelow. A comparison of A to B and C to D shows that using the structuredthermoplastic does not increase laminate thickness even when curing withvacuum pressure only.

TABLE 1 Laminate Cured ply thickness thickness cpt Laminate (mm) (mm) A1.70 0.147 B (Comparative) 4.77 0.149 C 4.51 0.141 D (Comparative) 4.410.138 E (Comparative) 4.78 0.149 F (Comparative) 4.99 0.156CAI Measurements

Laminates A-F were tested for compression strength after impactaccording to test method AITM 1.0010 (EN6038). Laminates according tothe invention (A and C) can be seen to have significantly improved CAIstrengths over laminates without an open-structured sheet (B and D).Laminate A, which uses the same IM7-12K fibres as laminates E and F, hasa significantly higher CAI value. This demonstrates the advantage ofusing structured thermoplastic polymer layers in accordance with theinvention instead of thermoplastic particles when curing out of theautoclave.

TABLE 2 Impact CAI Strength (MPa) Energy (J) A B C D E F 10 411 348 — —338 338 20 411 276 — — 309 307 25 326 259 — — 272 255 30 339 223 277 184280 246 40 285 217 — — 225 235 50 250 180 — — 215 204Other Mechanical Properties

Other composite properties for materials C and D were tested accordingto the table below. The results demonstrate that the veil does not haveany detrimental effect on these other properties.

TABLE 3 Test Laminate D Test Standard Conditions Laminate C(Comparative) OHC strength AITM 70° C. 246 230 (MPa) 1.0008 Wet OHTstrength AITM 70° C. 339 352 (MPa) 1.0007 Wet

Example 7 (Hybrid-Polyamide Veil Coated with PES)

The hybrid layer or membrane made in accordance with the processdescribed in FIG. 6 was incorporated into a laminate in resin transfermoulding. In this moulding, Hexflow® RTM6 (from Hexcel Corporation)thermoset epoxy resin was used as the matrix resin. Preforms wereassembled consisting of 16 plies of 268 g aramid unidirectional fibrewith a layer of hybrid non-woven material between each ply. This preformwas then placed into a heated mould cavity (4 mm thickness) and Hexflow®RTM6 was injected at a temperature of 100° C. After resin injection thetemperature of the mould was raised to 180° C. and the laminate curedfor 2 hours at 180° C. before cooling and demolding.

The laminates provided were tested for compression after impact (CAI)and bearing strength and the results are shown Table 4.

TABLE 4 Bearing CAI Strength Laminate Type (Mpa) (Mpa) RTM6 thermosetepoxy 169 859 RTM6 thermoset and PES/aramid membrane 206 897

The incorporation of the PES coated polyamide hybrid layer into thecomposite material having the Hexflow® RTM6 thermoset resin matrixresulted in improvements in both CAI and bearing strength.

Example 8 (Polymer Film or Membrane with Epoxy and Curative)

A polymer dope was prepared from 98 grains of dimethylsulfoxide, 2 ml of10%-aqueous 20 sodium acetate solution and 25 grams of polyethersulfone.0.72 g of tetraglycidyl derivative of diaminodiphenylmethane, which isan epoxy resin sold as Araldite MY72l by Huntsman (Basel, Switzerland)and 0.53 g of 4,4-methylenebis(2-methyl-5-isopropylaniline), which is acuring agent for the epoxy resin that is sold by Lonza (Basel,Switzerland). The epoxy resin and curing agent were incorporated into 10ml of the dope and cast onto a stippled surface and then precipitatedfrom water. When washed and dried, the layer comprised a white, strongveil which when heated at 180° C. for two hours gave a clear, flexible,tough epoxy-reinforced PBS film or membrane.

The membranes described herein are free from fibre crossover points thatact as stress concentrators. It is intended that the membranes describedherein form part of a curable composite assembly. Therefore, by solubleit is meant at least partially soluble in a matrix resin which formspart of the assembly. Furthermore, the membrane is soluble only duringcure of the assembly.

Therefore, the membranes describe herein may be made from any suitablethermoplastic polymer. The polymer is limited only by its ability todissolve in the matrix resin during curing of said resin. Such matrixresins, as will be described hereinafter, are typically thermosettingresins.

Suitable thermoplastic polymers for use in the membrane of the presentinvention include any of the following either alone or in combination:polyether sulfone (PBS), polyetherethersulfone (PEES), polyphenylsulfone, polysulfone, polyimide, polyetherimide, aramid, polyamide,polyester, polyketone, polyetheretherketone (PEEK), polyurethane,polyurea, polyarylether, polyaylsulfides and polycarbonates.

Although the terminal group of the chosen polymer is not material to theinvention, reactive terminal groups are preferable in order to link thepolymer membrane and the matrix resin upon curing. Forming such a linkenhances the toughness of the cured composite assembly of which themembrane forms parts.

Preferably, the polymers of the present invention comprise at least onelinking member in order to link together the polymer membrane and thematrix resin upon curing. The linking member may be a particularterminal group or a side-chain functional group present on the polymer.

Suitable terminal and/or side-chain groups include any of the followingeither alone or in combination: hydroxyl, chloro, amino, isocyanato,cyonato, glycidyl, carboxyl, nitro and sulfato.

The polymer preferably has a number average molecular weight in therange of from 100 to 10,000,000 daltons and most preferably from 10,000to 1,000,000 daltons.

The membranes described herein may comprise a plurality of apertures ofvarying shapes so as to provide an openwork structure. Advantageously,the shape and frequency of the apertures can be tailored to the specificphysical characteristics e.g. viscosity, of the resin in order that theresin can flow through the apertures and be evenly distributed throughthe composite assembly.

The presence of apertures eliminates the need to mechanically perforatea continuous membrane in order to facilitate the flow of resintherethrough. The membranes made in accordance with the matrix anddescribed herein are stronger than mechanically perforated membranessuch that the risk of tearing the film during its production isminimised.

The apertures of the present invention may take a variety of patternformats. Non-limiting examples include, fabric (so-called linenpattern), lattice, trellis, mesh, mat, net, stipple, pyramidal,hexagonal, rhomboid, hammered, knurled, lozenge and grid.

More than one pattern format may be present in a membrane in order toachieve particular properties. For example, combining pattern formatsmay result in a preferential strength or infusibility in a particularregion of the membrane.

These varying patterns allow a membrane to be tailored to a particularsystem. The pattern selection is influenced by the viscosity of thematrix resin of the composite assembly. Clearly, a lower viscosity resincan pass through a smaller sized aperture and vice versa. In the case ofa resin soluble membrane, the infusion of a laminate containing amembrane described herein must be performed at a temperature below thedissolution temperature of the membrane, to avoid washing or viscositybuild up.

Although the membranes described hereinbefore have a fabric likeappearance, such membranes are non-fibrous. It is this non-woven naturewhich gives the membranes described herein a significant advantage overprior art fibrous materials. The membranes described herein do notcontain fibre cross-over points, as with woven fabrics, which causelocal deformation of the reinforcing fibres. Furthermore the membranesdescribed herein are of a uniform thickness. This is a particularadvantage for vacuum infusion processes as it provides reducedinterlayer thickness and enables higher volume fraction fibre compositesto be produced, as required by the aerospace industry.

The membranes described herein preferably have an areal weight in therange of from 3 to 50 gm⁻² and more preferably in the range of from 5 to25 gm⁻².

The membranes described herein are preferably such that from 1 to 90% ofthe surface area of the membrane is apertured, so as to be open. Morepreferably from 5 to 50% of the surface area of the membrane isapertured so as to be open.

The membranes described herein may comprise one or more additionalcomponents which are useful in the cured composite of which the membraneforms a part. Such components include, but are not limited to, any ofthe following either alone or in combination: toughening particles,fillers, intumescent agents, flame retardants, pigments, conductingparticles, short fibres, resins and curing agents.

Toughening particles may include any of the following either alone or incombination: polyamides, copolyamides, polyimides, aramids, polyketones,polyetheretherketones, polyesters, polyurethanes, polysulfones, highperformance hydrocarbon polymers, liquid crystal polymers, PTFE,elastomers and segmented elastomers.

Preferably, toughening particles constitute from 0.1% to 80% by weightof the total weight of the membrane and most preferably from 1% to 50%by weight of the total weight of the membrane.

Suitable fillers may include any of the following either alone or incombination: silicas, aluminas, titania, glass, calcium carbonate andcalcium oxide. Preferably, fillers constitute from 0.1% to 30% by weightof the membrane.

Suitable conducting particles include any of the following either aloneor in combination: silver, copper, gold, aluminium, nickel, conductinggrades of carbon, buckminsterfullerene, carbon nanotubes and carbonnanofibres. Metal coated fillers may also be used, for example nickel,coated carbon particles and silver coated copper particles.

Preferably the conducting particles constitute from 0.1% to 98% byweight of the total membrane weight and more preferably from 10% to 90%by weight of the total membrane weight.

During cure of a composite assembly the polymeric material for themembrane dissolves leaving the insoluble moieties precisely located, ina depth wise sense, within the cured assembly.

Suitable insoluble moieties for inclusion in the membrane include any ofthe following either alone or in combination: intumescents, pigments,mould release agents, nano sized particles and conducting particles.

According to a second aspect of the present invention there is providedthe use of an at least partially soluble, non-fibrous, aperturedmembrane comprising at least one soluble thermoplastic polymericmaterial for delivering at least one insoluble moiety to a preciselocation within a cured composite assembly and wherein said membrane hasa discrete porous structure.

The present invention further seeks to provide a composite assemblyhaving excellent toughness properties whereby it is possible to controlthe location of toughening agent within the assembly.

According to a further aspect of the present invention there is provideda curable composite assembly comprising a polymeric matrix resin, afibrous reinforcement material and at least one non-fibrous, at leastpartially soluble, apertured, membrane wherein said membrane comprisesat least one thermoplastic polymeric material soluble in the matrixresin and wherein said membrane has a discrete porous structure.

The assemblies of the present invention may be prepared by either director indirect processes. That is, the assemblies of the present inventionmay be prepreg, an indirect process, prepared by incorporating thelayers into the assembly during the prepregging. Alternatively they maybe assemblies prepared by direct processes, such as RTM, VaR.TM or RFL

For prepreg made by a solvent process, the membrane would be interleavedor consolidated onto the reinforcement just prior to final wind-up ofthe product. For hot melt prepreg processes, the membrane can either belaminated onto the reinforcement prior to final wind-up or it could bepositioned on or into the reinforcement prior to combining thereinforcement with the matrix film. Such techniques are well known tothose skilled in the art.

In the case of RTM, VaRTM and RFI processes, assemblies are prepared byapplying the membranes described herein to the dry fibrous material ofthe preforms. There are some reinforcement materials, such as multiaxialtextiles where it is possible to locate the membrane in betweenindividual layers comprising the textile. The matrix resin is of aviscosity such that, during the resin injection stage, the resin passesthrough the membrane into the fibrous material. These technologies aredescribed in chapter 9 of “Manufacturing Processes for AdvancedComposites”, F.C. Campbel Elseveir, 2004.

The preferred thennoset matrices for RIM processes are epoxy orbismaleimide (BMI) with suitable epoxy examples being HexFlow® RTM 6 orRTM 120. A typical BMI matrix is HexFlow® RTM 65 L HexFlow® VRM 34 maybe used for Vacuum-assisted Resin Transfer Moulding (VaRTM)applications. All of the above materials are available from HexcelComposites, Duxford, UK.

The reinforcement fibres can be selected from any of the followingcommercially available high performance fibres which may be used aloneor in combination: aramid (e.g. Kevlar™), glass, carbon, ceramic, hemp,or polyolefin. Carbon fibres are the preferred material, particularlystandard or intermediate modulus fibres of between 3000-24000 filamentsper fibre tow. The desirable reinforcement form is a woven ornon-crimped textile structure of between 150-1000 gm⁻² fibre arealweight. Typical weave styles include plain, satin and twill weaves.Non-crimped or multiaxial reinforcements can have a number of plies andfibre orientations such as +45/−45; 0/+45/−45; 0/+45/−45/90. Such stylesare well known in the composite reinforcement field and are availablefrom a number of companies including Hexcel Reinforcements,Villeurbanne, France.

The present invention also provides a method by which the membranesdescribed herein can be made.

Therefore, according to a still further aspect of the present inventionthere is provided a method for the preparation of a non-fibrous, atleast partially soluble, apertured, porous membrane comprising at leastone soluble thermoplastic polymeric material said method comprising thesteps of:

-   -   a) preparing a polymer dope solution comprising said        thermoplastic polymeric material in solvent;    -   b) casting said dope solution;    -   c) bringing the cast dope solution into contact with a        coagulation means so as to form a membrane;    -   d) removing at least some of the solvent from the membrane; and    -   e) drying the membrane.

The method of the present invention is also preferable to prior artcasting processes as these do not allow for the formation of preciselydefined microporous structures. The polymer dope solution is prepared bydissolving the polymeric material in a solvent or mixture of solvents.Any solvent conventionally used for preparing polymer solution may beused. However, preferred solvents are those which are substantiallyentirely miscible with water. More preferably suitable solvents areaprotic solvents and most preferably suitable solvents are polar aproticsolvents.

Therefore, suitable solvents for use in the method of the presentinvention include any of the following either alone or in combination:dimethyl sulfoxide, dimethyl formamide, dimethylacetamide,1-methylpyrrolidone, tetramethylurea, gamma-butyrolactone, propylenecarbonate and ethylene carbonate.

The solvent or solvent mixture referred to above may also comprise aco-solvent in order to modify the solution characteristics of thesolvent.

Suitable co-solvents include any solvent which is completely misciblewith any of the aforementioned solvents. Suitable examples includeglycol ethers and alcohols. The solvent for use with the presentinvention may contain other soluble additives to aid in the coagulationand washing stages of the process. For example, lithium chloride,potassium chloride, calcium chloride, sodium acetate, surfactants suchas sodium dodecylsulfate and the like.

The polymer dope solution of the present invention preferably comprisesfrom 5% to 90% by weight of polymer, more preferably from 5% to 50% byweight of polymer and most preferably from 10% to 35% by weight ofpolymer.

The polymer dope solution may also comprise one or more additionalcomponents useful in a cured composite. Such additives include, but arenot limited to, any of the following either alone or in combination:toughening particles, fillers, intumescent agents, flame retardants,pigments, conducting particles, short fibres, resins and curing agents.In, fact, any material that is insoluble in the coagulation and washingliquids can be added to the polymer dope solution.

Examples of suitable additional components are described above inrespect of the membrane per se.

Chemically reactive components may also be included in the polymer dopesolution. Suitable reactive components are preferably insoluble in thecoagulation and washing liquids used in the process for the presentinvention. In practice, most epoxy resins, cyanate ester resins and mostcuring agents used in thermosetting systems are very insoluble in waterand are therefore suitable for use with the present invention.

The polymer dope solution may be cast onto a substrate or directly ontoa patterned roller, such as a gravure roller such as those used inprinting processes.

Suitable substrates include sheet material comprising polyethylene andcopolymers of ethylene, oriented polypropylene, polypropylene andcopolymers of propylene, polyamide, polyester and similar materials. Thesubstrates may be untreated or may be surface coated. Suitable surfacecoating materials include silicone release compounds with polyethyleneand low density polyethylene being particularly preferred.

Alternatively, the substrate may be porous and in the form of one ormore continuous belts. These substrates are well known in thepapermaking industry and may comprise a wire mesh, fibre mesh, felt orsimilar. Such an arrangement allows for greater washing efficiency andmore rapid manufacture with reduced tendency for the film material tobreak.

In order to provide an apertured membrane of a particular open workpattern the substrate is suitably embossed in order to create saidpattern.

Following casting the material is contacted by a coagulation means inorder to form a film. Said coagulation means may be a coagulation bathcomprising a coagulation liquid. The coagulation liquid may be water oranother liquid such as methanol, ethanol, propanols, acetone, and theiraqueous mixtures. During coagulation the polymer separates into amembrane having a microporous structure.

After coagulation, the polymeric membrane is passed through at least onewash bath in order to remove any coagulation liquid from the membrane.The wash bath comprises water which optionally comprises additives andsolvents such as alcohols to increase the rate of washing, enzymes orother agents to accelerate the degradation of extracted solvents,wetting agents and the like. The washing stage may advantageously beconducted using warm water (up to 50° C.) and/or agitation to furtherincrease the rate of solvent removal.

Other methods for increasing the rate of solvent removal may beemployed, for example ultrasonics.

For the manufacture of a membrane having an interconnected microporousstructure it is preferable to remove the majority of the solvent fromthe membrane during the washing stage. Following washing it ispreferable that the membrane comprises a maximum of 10% solvent withrespect to the total membrane weight and it is more preferable that themembrane comprises a maximum of 5% solvent with respect to the totalweight of the membrane. Further solvent may be removed during the dryingprocess.

For the manufacture of a membrane having a substantially discretemicroporous structure it is preferable to remove up to 90% of solventfrom the membrane. If too much solvent is removed, a membrane having adiscrete pore network cannot be produced. In this case, during thesubsequent drying process the interconnecting sub-micron pores formedduring coagulation are converted to micron sized discrete pores.

Following washing the membrane is dried. Drying is preferably achievedusing one or move ovens. That said the membrane can be dried at roomtemperature i.e. 20-25° C. Typically, drying takes place at atemperature within the range of 70° C. to 200° C. However, the precisetemperature is dependent upon the nature of the polymer from which themembrane is made and the substrate etc. Clearly, drying the membrane atan elevated temperature accelerates the drying procedure.

In a further embodiment of the invention, the membrane is made bycasting the polymer dope onto a pre-existing veil.

According to a further aspect of the present invention there is providedan at least partially soluble, apertured membrane, comprising at leastone soluble thermoplastic material secured to a substrate, thethermoplastic material having a discrete porous structure.

The preferably insoluble non-woven substrate provides strength andenables low weights of soluble polymer to be readily introduced into acomposite. The resin interface between the substrate veil material andresin matrix is toughened, thereby imparting additional toughness to thefinal composite part.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited by the above-describedembodiments, but is only limited by the following claims.

What is claimed is:
 1. A method for forming a film to be located betweenthe fibrous layers of a composite material, said method comprising thesteps of: a) dissolving polyethersulfone or polyetherimide in a solventto form a polymer dope; b) dispersing an epoxy resin and a curing agentfor said epoxy resin in said polymer dope to form an epoxy-enrichedpolymer dope; c) casting said epoxy-enriched polymer dope onto a surfaceto form a dope layer, said dope layer being free of fibers; d) removingthe solvent from said dope layer to form a fiber free veil; and e)heating said fiber free veil to form said film.
 2. A method according toclaim 1 wherein the step of removing the solvent from said dope layercomprises the steps of washing and drying said dope layer.
 3. A filmthat is made according to the method of claim
 1. 4. A composite materialcomprising a preform for use in resin transfer moulding wherein saidpreform is placed in a mould cavity and uncured resin is injected intosaid preform within said cavity, said preform comprising a plurality offibrous layers comprising woven or unidirectional fibers wherein a filmaccording to claim 3 is located between each of the fibrous layers.
 5. Acomposite material comprising a prepreg comprising a thermosetting resinand a plurality of fibrous layers comprising woven or unidirectionalfibers wherein a film according to claim 3 is located between each ofthe fibrous layers.
 6. A composite part that comprises a preformaccording to claim 4 that has been infused with a thermosetting resinand cured to form said composite part.
 7. A composite part thatcomprises a prepreg according to claim 5 that has been cured to formsaid composite part.